Icon Set Creator lets you quickly create icon assets for your apps: Just drag your high-resolution artwork (preferably, a PNG file at 1024x1024) into the app's window, chose the target platform, select a directory for exporting the results, and you're set. The currently supported formats are iOS, macOS, watchOS, and iMessage Apps.

Ideally, you would want to have your designer fine-tune each icon size by hand, for best visual results in terms of detail, pixel alignment, etc.
But if you just want to quickly fill all the "buckets" in the AppIcon entry of your asset catalog, this simple tool can help you.